Billions of pounds of post-consumer carpet waste are generated each year in the United States. Landfilling is not an environmentally friendly solution since carpet fibers, like many other synthetic polymers, are not biodegradable. Furthermore, the cost of disposal is increasing due to efforts to make landfills more environmentally secure and to preserve limited capacity. Environmental concerns and governmental regulations have spurred efforts to recycle as much of the non-biodegradable synthetic polymer waste stream as practicable.
In a post-consumer carpet waste there are generally three predominant components: backing fiber, face fiber, and non-fibrous backing materials. Polypropylene is present in the primary and secondary backing of most carpets, generally in the form of woven ribbons or nonwoven fabric. In addition, polypropylene fiber is sometimes found as the face fiber. The most common face fibers are the polyamides, Nylon 6 and Nylon 6,6. Backing compositions generally contain binders such as SBR latex and fillers such as calcium carbonate. Commercial carpet may employ mixed polyolefin, polyvinyl chloride, or polyurethane non-fibrous backing components.
The fiber component of the overall post-consumer carpet waste stream would be expected to consist of approximately 40% Nylon 6; 40% Nylon 6,6; and 10% Polypropylene. Sorting carpet pieces according to face fiber type before the pieces are shredded, further size-reduced, and separated into a fibrous component and a non-fibrous binder and dirt component, can yield segregated commingled fiber wastes composed of about 80% Nylon 6 or Nylon 6,6 and about 5% to 10% Polypropylene. This amount of Polypropylene polymer in Polyamide degrades the physical properties of the Polyamide polymer to such an extent that it is unsuitable for virtually all typical Nylon 6 or Nylon 6,6 applications. Thus, the utility of Nylon 6 and Nylon 6,6 polymers recovered from waste is, to a great extent, dependent upon the absence of polyolefin polymer contaminants.
U.S. Pat. Nos. 5,240,530 and 5,288,349 (Fink) teach a carpet recycling and recovery method in which portions of the carpet are initially ground and physically separated before being remelted for a feedstock. Carpets containing different types of polymers are ground mechanically to reduce fiber length to less than ±4 inch, then fibers of polypropylene, polyester, and polyamide are claimed to be separated on the basis of specific gravity when suspended in water. No specific extraction steps are disclosed.
U.S. Pat. No. 6,155,429 (Clark) teaches that enhanced separation of light and heavy particles from an aqueous suspension in a hydrocyclone by injection of air into the slurry upstream of the hydrocyclone is useful in the recycling of Nylon face fiber from post-consumer carpets. The carpets must be converted to a form that can be slurried and processed in a hydrocyclone after first being sorted by face fiber type. Carpets of like polymer face fiber (Nylon 6,6 or Nylon 6) are size-reduced by suitable means, such as with a hammermill, to pieces suitable for further size-reduction. After sifting to remove filled binder and loose dirt, the remaining fibrous product is further size-reduced, such as with a cutter, to reduce the fibrous material to a particle size such that it passes through a screen with holes less than or equal to about 0.125 inches (0.32 cm) and then admixed with water. The specific gravity of nylon is generally about 1.2 which is greater than the specific gravity of water (1.0) which is greater than the specific gravity of polypropylene (0.9). The apparent specific gravity of these materials is stated to be dependent upon the crystallinity and processing of said materials, so separation efficiency would be expected to be variable.
Solvents such as octane have been proposed to dissolve Polypropylene bonded to Polyamide fibers without altering the Polyamide fibers. The Polypropylene is separated from the solvent by cooling with subsequent filtration (Tselishcheva et al.; International Polymer Science and Technology; 29, No. 8, p.T/55–6; 2002). This process would appear to be unsuitable for a waste containing a substantial proportion of Polyamide because of the difficulty of effectively removing Polypropylene-laden solvent from a substantial mass of Polyamide fibers.
U.S. Pat. Nos. 5,198,471 and 5,278,282 (Nauman) teach selective dissolution of individual polymers from a mixed polymer waste stream. Polymers are dissolved in a solvent at progressively higher temperatures with multiple extractions of solvent solution so that individual pure polymers can be precipitated from solution. Each polymer is dissolved at a temperature near, but not exceeding, the melting temperature of the polymer. Thus, the polyolefins would be dissolved prior to dissolution of either Nylon 6 or Nylon 6,6 which have higher melting temperatures than polypropylene or the other polyolefins. Once again, this process would appear to be unsuitable for a waste containing a substantial proportion of Polyamide fibers because of the difficulty of effectively removing Polypropylene-laden solvent from a substantial mass of Polyamide fibers.
U.S. Pat. No. 5,430,068 (Subramanian) teaches a process for recovering polyamide from admixtures with foreign materials by dissolving the polyamide, at an elevated temperature substantially below the melting temperature of Polypropylene, in a solvent selected from the group consisting of a substantially anhydrous ethylene glycol, propylene glycol, and aliphatic carboxylic acid having from 2 to 6 carbon atoms, filtering, then combining the polyamide solution with an additional quantity of substantially the same solvent at a lower temperature to cause the polyamide to precipitate. The named solvents may react with components of the carpet backing such as calcium carbonate. Rapid cooling by addition of substantial quantities of cool solvent is necessitated by degradation of polyamide when held in the hot solvent.
U.S. Pat. No. 5,898,063 (Stefandl) teaches a recycling and recovery process for waste carpet employing a solvent such as ethylene glycol, propylene glycol, glycerol and various mixtures of these solvents, or, alternatively, an organic formate, hydrochloric acid, formic acid, methanol, nitric acid, glacial acetic acid, fluorinated alcohols, m-cresol, phenolic compounds, chloroform-methanol, methanol-lithium chloride, potassium thiocyanate, benzyl alcohol, butane diol 1,1, dimethyl sulfoxide, triethylene glycol, or tetraethylene glycol. Nylon 6 and Nylon 6,6 are taught to be soluble in each of these solvents at various elevated temperatures. Once again, these solvents may react with components of the waste carpet backing present with the Nylon 6 and Nylon 6,6 or degrade the dissolved polyamide polymer, additionally, most present environmental, and worker health and safety hazards. U.S. Pat. No. 6,140,463 (Stefandl) teaches recovery of a purer Nylon polymer from carpet by dissolving and precipitating the Nylon polymer at least twice utilizing the same solvents described in U.S. Pat. No. 5,898,063 (Stefandl).
U.S. Pat. No. 5,908,164 teaches a process for the physical separation and recovery of fractional components of used carpet involving pre-cutting and preferably pre-soaking of the used carpet into appropriate sized pieces followed by the introduction of the pre-conditioned used carpet pieces into a cyclonic comminuter which reduces the carpet pieces into fractional components.
U.S. Pat. No. 6,126,096 teaches a process for physical separation of the components of waste carpet including soaking and chilling appropriately sized carpet pieces to or below freezing to stiffen the backing material and enhance the comminution process, then collecting the components from the respective discharges from the cyclonic comminuter, washing, and separating by carding, static charges, pressure gradients and the like.
U.S. Pat. No. 5,994,417 teaches a process for recovering polymers from commingled materials by selectively dissolving the polymer in a solvent and then contacting the solution with an anti-solvent comprising a compressed fluid, near, at or above its critical pressure into which the solvent is soluble but into which the polymer is insoluble. The anti-solvent is selected from the group consisting of ethane, propane and carbon dioxide.
All of these prior art approaches to the separation of Nylon 6 or Nylon 6,6 polyamide polymers from the polypropylene polymer usually found in the backing of carpets suffer from shortcomings, thus an unmet need exists for an environmentally benign, inexpensive means of purifying polyamide polymer recovered from commingled polyamide and polyolefin wastes.